Transition-metal-substituted polyoxometalates as water oxidation
catalysts Open Access

Zhu, Guibo
(2013)

Abstract

Several new polysilicotungstates substituted with earth-abundant
elements (Co and Ni),
K10.2Na0.8[{Co4(µ-OH)(H2O)3}(Si2W19O70)]·31H2O,
K5Na4H4[{Na3(µ-OH2)2Co2(µ-OH)4}(Si2W18O66)]·37H2O,
K6Na3[Na(H2O){Co(H2O)3}2{Co(H2O)2}(Si2W18O66)]·22H2O
and
K10H2[Ni5(OH)6(OH2)3(Si2W18O66)]·34H2O,
have been synthesized and characterized by X-ray crystallography,
infrared, elemental analysis, UV-vis spectroscopy and cyclic
voltammetry. They can effectively catalyze water oxidation in a
photo-driven system with [Ru(bpy)3]2+ as
photosensitizer and Na2S2O8 as the
sacrificial electron acceptor. Several lines of evidence show that
none of these complexes decompose to insoluble metal oxide under
the harsh catalytic conditions. Although
[{Co4(µ-OH)(H2O)3}(Si2W19O70)]11-
slowly hydrolyzes in aqueous solution, it is quite probable that
the initial complex is the real catalyst, because the isolated
hydrolysis products show lower WOC activity. Because of their low
solubility in aqueous solution,
[{Na3(µ-OH2)2Co2(µ-OH)4}(Si2W18O66)]13-,
[Na(H2O){Co(H2O)3}2{Co(H2O)2}(Si2W18O66)]9-,
and
[Ni5(OH)6(OH2)3(Si2W18O66)]12-
form solution ⇔ [Ru(bpy)3]n+-POM
complex equilibria and remain molecular during the catalytic
process.

A dodecazinc silicotungstate
K20Na2[Zn6(OH)7(H2O)(Si2W18O66)]2·34H2O
has been synthesized and characterized. DFT B3LYP calculations give
HOMO-LUMO and (HOMO-1)-LUMO energy gaps of ~3.65 and 3.91 eV, which
are larger than in ZnO (band gap = 3.35 eV).

The catalytic activity of tetra-n-butylammonium salt of
[{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10-,
whose water soluble salt is a water oxidation catalyst, was
explored in oxidation of 3,5,3',5'-tetrabutyl-biphenyl-4,4'-diol by
dioxygen. Alcohols are stoichiometricaly oxidized under Ar by this
compound to aldehydes/ketones in acetonitrile solution. The
reaction kinetics and stoichiometry were studied by GC, NMR and
UV-vis spectroscopies. The reaction kinetics are very complex, the
reaction proceeds in two steps with the second step being
significantly slower. Several reaction mechanisms were analyzed by
fitting of the reaction kinetic curves obtained from changes of
UV-Vis spectra versus time.

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